In a turbomachine, such as a gas turbine engine, air is pressurized in a compressor then mixed with fuel and burned in a combustor to generate hot combustion gases. The hot combustion gases are expanded within the turbine section where energy is extracted to power the compressor and to produce useful work, such as powering a propeller for an aircraft in flight or turning a generator to produce electricity. The hot combustion gas travels through a series of turbine stages. A turbine stage may include a row of stationary vanes followed by a row of rotating turbine blades, where the turbine blades extract energy from the hot combustion gas for powering the compressor and providing output power. Since the turbine blades are directly exposed to the hot combustion gas, they are typically provided with internal cooling circuits which channel a coolant, such as compressor bleed air, through the airfoil of the blade and through various film cooling holes around the surface thereof. One type of airfoil extends from a root at a blade platform, which defines the radially inner flowpath for the combustion gas, to a radially outer cap or blade tip section, and includes opposite pressure and suction sides extending axially from leading to trailing edges of the airfoil. The cooling circuit extends inside the airfoil between the pressure and suction sides and is bounded at its top by the blade tip section.
The gas turbine engine efficiency is, at least in part, dependant upon the extent to which the high temperature gases leak across the gap between the turbine blade tips and the seals or shrouds which surround them. The leakage quantity is typically minimized by positioning the radially-outward blade tip section in close proximity to the outer air seal. However, differential thermal elongation and dynamic forces between the blade tip section and outer air seal can cause rubbing therebetween. Also, it should be noted that the heat load on the turbine blade tip section is a function of leakage flow over the blade tip section. Specifically, a high leakage flow will induce a high heat load to the blade tip section, such that gas leakage across the blade tip section and cooling of the blade tip section have to be addressed as a single problem. In a typical construction, the blade tip section of an airfoil may be provided with a squealer tip rail extending radially outwardly a short distance from the blade tip section, and extending substantially completely around the perimeter of the airfoil to define an inner squealer tip pocket facing radially outwardly. The squealer tip rail is located radially closely adjacent to a stationary outer seal wall, or outer turbine shroud, to provide a relatively small clearance gap therebetween to seal or restrict the flow of gas across the blade tip section.
The squealer tip rail is a solid metal projection of the airfoil, and is directly heated by the combustion gas which flows thereover. In addition, a vortex flow of hot gases is typically formed on the suction side of the airfoil adjacent the blade tip, where the size of the vortex generally increases toward the aft end, i.e., the trailing edge, of the airfoil. The squealer tip rail is cooled by a cooling fluid, such as air, channeled from an airfoil cooling circuit to the blade tip section to convect heat away the area of the squealer tip pocket. Convective cooling holes may be provided in the squealer tip pocket located along the squealer tip rail. In addition, heat from the squealer tip rail may be conducted into the squealer tip section and convected away internally of the airfoil by the cooling fluid channeled through the internal cooling circuit. The squealer tip section, including the squealer tip rail, typically operates at temperatures above that of the remainder of the airfoil and can be a life limiting element of the airfoil in a hot turbine environment. In particular, it is known in the art that the portion of the airfoil located at the intersection of the pressure side airfoil surface and the blade tip section is subject to very high heat loads and accordingly is more likely to experience thermal distress. Accordingly, there is a continuing need to provide effective cooling to the surfaces of the blade tip section, and particularly to the squealer tip rail.